1. Field of the Invention
This invention relates to a radiation image storage panel employed in a radiation recording and reproducing method utilizing a stimulable phosphor, and more particularly, to a radiation image storage panel comprising a support and a colored phosphor layer.
2. Description of Prior Arts
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and a radiographic intensifying screen.
As a method replacing the above-described radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock. The transparent protective film is generally formed on the phosphor layer by combining a thin film which is beforehand prepared, with the phosphor layer through an adhesive agent.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with stimulating rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or having radiated from an object is absorbed by the phosphor layer of the radiation image storage panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission (light emission) by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to obtain electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very useful for obtaining a radiation image as a visible image as described hereinbefore, and it is desired for the radiation image storage panel employed in the method to provide an image of high quality (high sharpness, high graininess, etc.).
The sharpness of an image in the radiation image recording and reproducing method generally depends not on the spread of the light emitted by the stimulable phosphor within the radiation image storage panel, but on the spread of stimulating rays therewithin. That is because the radiation energy-stored image recorded on the panel is sequentially detected. For enhancing the sharpness of the image provided by the panel, it has been proposed to color the panel with a colorant capable of absorbing a portion of the stimulating rays for the stimulable phosphor contained therein (Japanese Patent Provisional Publication No. 55(1980)-163500, which corresponds to U.S. Pat. No. 4,394,581, etc.). The publication discloses that the sharpness of the image is effectively improved when the phosphor layer of the panel is colored with the colorant.
However, when a pigment is employed as the colorant to color the radiation image storage panel, it was extremely difficult to obtain a coating dispersion which comprises phosphor particles and the pigment homogeneously dispersed in a binder solution even by use of a dispersing apparatus, etc. A dispersing agent such as phthalic acid, stearic acid or caproic acid has been conventionally known for employment in the coating dispersion to improve the dispersibility of the phosphor particles therein. Even when a dispersing agent is added to the coating dispersion to disperse mechanically, the homogeneous coating dispersion containing the phosphor particles and pigment can not be easily obtained.
A colored phosphor layer formed by applying the heterogeneous coating dispersion onto a support shows portions where the phosphor particles and pigment densely exist and other poritons where they sparsely exist, because of the nonuniform dispersibility thereof. As a result, the stimulating rays are excessively absorbed by the condensed portions of the pigment so that the radiation image storaage panel having such colored phosphor layer gives an image on which a number of fine white spots are occurred.
In addition to the occurrence of the white spots owing to the agglomeration of the pigment, there is a tendency that the graininess of the image lowers because of the nonuniform dispersibility of the phosphor particles in the phosphor layer. Further, the poorly dispersed pigment can not satisfactorily bring about the enhancement of the sharpness of the image, though the phosphor layer is so colored with the pigment as to improve the sharpness of the image.